Reformation of the nuclear envelope (NE) around the segregated chromosomes is a key event at the end of mitosis. Defects in this key process may result in alteration of gene expression patterns and genomic instability. We have generated new information about the two major regulators of nuclear assembly, the GTPase Ran and the AAA-ATPase p97. Our specific aims are: 1. To understand how Importin b, a major RanGTP binding protein, regulates NE fusion and to identify the molecular targets with which it interacts. We will use a 2-color fusion assay and transmission electron microscopy (TEM) to characterize how Importin b inhibits the fusion of chromatin bound vesicles. The dynamics of NE tubule formation and reorganization will be visualized by live imaging on chromatin-coated glass slides. We will use biochemical fractionation methods and affinity chromatography to identify the binding partner(s) of Importin b. 2. To characterize NE membrane sealing and to identify the Ufd1/Np14 regulated NE fusion machinery. To understand p97/Ufd1/Np14-dependent formation of a closed NE, we will use TEM, a novel nuclear exclusion assay, and real time microscopy. To identify additional proteins that interact with Ufd1/Np14 and participate in NE sealing, we will use a recombinant form of Ufd1/Np14 complex as matrix for affinity chromatography. 3. To characterize a second GTPgS-sensitive step in NE formation. We will use the 2-color fusion assay and TEM to characterize this membrane fusion event. We will analyze a membrane-associated GTPase activity on chromatin-bound vesicles. To identify the nature of the additional GTPase(s) we will perform an RNAi screen in C. elegans to test if GTPases known to mediate intracellular membrane fusion (e.g. Rab GTPases) are involved in NE formation. As an alternative approach we will use photo-affinity methods, overlay assays and proteomic approaches to identify the second GTPase.